Semiconductors and semiconductor devices are ubiquitous in the modern world. Diodes, transistors, integrated circuits, light emitting diodes (LEDs), solar cells, semiconductor lasers, and the like are used in radios, televisions, computers, automobiles, home appliances, industrial equipment, and essentially anywhere information is stored, transmitted or controlled electrically. At present, semiconductor devices are nearly exclusively inorganic semiconductors, and almost exclusively silicon or gallium arsenide based. But silicon and other inorganic semiconductors are mechanically brittle, costly to make, purify, and process, and must be processed at high temperatures.
It would be desirable to have semiconductors and semiconducting devices that were flexible, for example, so that electrical circuits could be flexed during use and/or bent to conform to curved surfaces. It would also be desirable to have semiconductors and semiconducting devices that were easier and less costly to purify and process. And it would be desirable to have semiconductors and semiconducting devices that could be processed and fabricated at low temperatures, in particular at temperatures below the glass transition temperature of common plastics such as polyesters and polyamides, so that plastics such as these could be used as substrates on which to build semiconducting devices and circuits.
While semiconductor devices using organic instead of inorganic materials have been fabricated before, these devices consisted of only undoped organic compounds, or organic compounds doped with small, relatively mobile ions such as bromide anions or sodium cations. Doping with small, mobile ions is undesirable because the positive (donor) and negative (acceptor) dopants can meet and neutralize one another. On the other hand making diodes or p-n junctions with undoped semiconductors seriously limits the current that may be passed through such a device. Unlike conventional p-n junctions where a p-type semiconductor makes contact with an n-type semiconductor, junctions of undoped semiconductors do not form depletion layers or built in potentials—the basis for many p-n junction properties. This built in potential is essential for the operation of conventional semiconductor junction solar cells, for example.
There is a growing literature on organic transistors. These are all made with pure, undoped polymers, oligomers, or small molecules (see for example, C. D. Dimitrakopoulos and D. J. Mascaro, “Organic thin-film transistors: A review of recent advances,” IBM J. of Res. & Dev. 2001, 45, 11-27). Essentially, all reported organic transistors are of the field effect type, in which electrical potential on a gate electrode alters the number and path of charge carriers in a thin undoped organic layer, and thus the current flow between a source electrode and a drain electrode on opposite ends of the thin organic layer. Organic field effect transistors do not have a p-n junction and performance is improved if adventitious dopants are removed from the single organic layer.
Accordingly, it would be desirable to find organic materials and methods for fabrication of bipolar transistors.
There have been numerous reports of organic solar cells, which are often referred to as organic p-n junctions. While these solar cells produce photovoltages and photocurrents they are invariably fabricated with undoped polymers or undoped small molecules. Since they are undoped they are not true p-n junctions, and therefore not true analogs of silicon or inorganic p-n junctions. A true organic p-n junction would consist of an organic material doped with an acceptor (p-type) in contact with a piece of the same organic material doped with a donor (n-type), forming a p-n homojunction. Likewise, a true organic heterojunction would consist of an organic material doped with an acceptor (p-type) in contact with a piece of a different organic material doped with a donor (n-type). Contact between two undoped polymers is more correctly referred to as a polymer bilayer, and not a p-n junction. Polymer bilayers do not develop a space charge region or a built in potential, and have relatively high resistivity.
In a recent study on Organic Photovoltaic (OPV) cells Gregg and Hanna remarked, “To our knowledge, no homojunction OPV cells have been reported, probably due to the difficulty of doping organic semiconductors.” (B. A. Gregg and M. C. Hanna, J. Appl. Phys., 2003, 93, 3605-3614.) One of the difficulties referred to by Gregg et al. is the higher mobility of dopants in organic materials relative to inorganics. Another difficulty is the lack of organic doping agents that can reduce (n-type) or oxidize (p-type) an organic material and leave behind only a cation dopant (n-type) or an anion dopant (p-type) and no other byproducts.
It would be highly desirable to be able to prepare both n-doped and p-doped organic semiconductors, where the dopants are not mobile and do not diffuse during use or over the lifetime of the device. It would be further desirable to prepare true p-n junctions of organic materials, such that a built in potential is developed, and where dopants do not diffuse and neutralize each other. It would also be desirable to have organic diodes, transistors, LEDs, semiconductor lasers, photocells and solar cells based on such true organic p-n junctions.